The healing of ileal and colonic anastomoses is compared in rabbits. The intestinal segment that contains the anastomosis shows a temporary loss of strength, which is reflected in a decreased bursting pressure. This loss of strength is accompanied by a massive loss of collagen, measured as hydroxyproline, both in ileum and in colon. In ileum, hydroxyproline concentrations, expressed on a dry weight basis, are lowered by 30 per cent, one day after operation. Thereafter, they rise again, after seven days reaching a level that is 40 per cent enhanced as compared with unwounded tissue. Maximal decrease in colon, measured two days after operation, is 40 per cent. After seven days hydroxyproline levels are back at preoperative values. In colon, significant loss of hydroxyproline is also apparent in the intestinal segment proximal to the anastomosis. This phenomenon does not occur in ileum. These results clearly demonstrate that after ileal anastomosis a loss of collagen occurs similar to that in colonic anastomoses. The fact that the loss of collagen is less extensive and more rapidly restored may be important in explaining the lesser incidence of leakage encountered after surgery of the small intestine.